'Discrimination'

This research from the University of Paris-Saclay has measured the earnings difference in terms of women with and without children - and found it to be on average 3% per child.

Using 16 years of income data in private sector companies in France, the study found that mothers were likely to earn less, while men did not have any loss of income linked to parenthood.

The study examined the possible causes of the pay gap.

This included mothers making a trade-off between rates of pay and having more flexibility over hours and location.

There were also suggestions that such "family friendly" approaches were likely to come with a loss of earnings.

Another aspect of lower pay could be that time away for maternity leave could disrupt the trajectory of a career and that mothers could miss out on the training and experience needed for promotion.

But the study found that none of these possible causes explained the extent of the pay gap - and concluded that an underlying cause could be "discrimination" against mothers.

This could either be bias against mothers with young children not being accepted for jobs, or mothers being assigned to areas seen as less challenging but also less likely to receive bonuses and promotions.

With a long period of wage stagnation after the recession, there has been growing attention on the impact of people trapped in low-income jobs and "in-work poverty".